A variety of hydrophobic, aromatic, halogenated haptens have been used to produce defined antigens. Antibodies to many of these have been raised successfully. Procedures for characterization and insuring reproducibility of the antigens have been worked out. Utilization of the antisera in development of immunoassays is beginning. It is hoped that these assays will provide relatively inexpensive, specific and sensitive alternatives to assay methods presently available, as well as provide confirmatory techniques.